How to create a Raid Device using Madadm

How to create a Raid Device using Madadm

How to create a Raid Device using madadm in

linux rhel5 with level 5, 0 and 1

The following article explains what is Raid, what are important levels and how

to install and configure a raid device in a linux system using the software

mdadm This is tested in Redhat rhel5 and also works with other distributions

as fedora, centos etc

What is RAID?

RAID is redundant array of independent or inexpensive disks It is mainly used for data protection It protects our data storage from failures and data loss All storage units now use raid technology

It has following uses

1 Data protection

2 Increasing performance

Types of RAIDs:

There are alot of levels of RAIDs But the main levels are

1 Level 0 or Striping

2 Level 1 or Mirroring

3 Level 5 or Striping + Parity

Level 0:

It is also known as striping You know hard disk is a block device Datas are read and written from/to it by blocks

Suppose we have data block as below

1 0 1 1

Suppose each bit takes one cpu clock cycles for writing into a disk Total, it will take 4 cpu clock cycles

With stripping:

In striping we use "N" number of hard disks RAID devides the data block by

"N" and writes each part to each disk in parallel

If we have 4 hard disks, It'll take only one cpu clock cycle if we use Level 0 RAID

Raid 0 is best for where more writing take place than reading But is insecure

as there is no recovery mechanisms

Level 1:

Also known as Mirroring One is the exact replica of other Whatever writing to master disk will be written in to mirror disk also Reading can be performed fro each disk simultaneously, thus increasing the read performance

But can be utilize only 50% of the total size

Level 5:

It is a combination of striping and parity Need at least three hard disks Both parity and data are distributed in all If one hard disk fails, data on that can be regenerated by the data and parity information in the other two hard disks

###RAID###

Raid 5 :need 3 disks

Raid 0 :need 2 disks

Raid 1 :need 2 disks

first partition disks

***RAID 5***

Here we'll show how to create a Level 5 raid device Here we use three

partitions /dev/sda5 /dev/sda6 /dev/sda7 Keep in mind that, in real industry it'll

be three different hard disks

This following command will create a raid device /dev/md0 with level 5

#mdadm create /dev/md0 level=5 raid-devices=3 /dev/sda{5,6,7}

Formatting the raid device

#mke2fs -j /dev/md0

Creating mount point

#mkdir /raid5

Mounting the raid device to the created mount point

#mount /dev/md0 /raid5

Making the mount permanent by adding it in fstab

#vi /etc/fstab

/dev/md0 /raid5 ext3 defaults 0 0

:wq

#mount -a

Test read and write permissions And if you want you can check whether the raid is working by failing or deleting any partition it consists of

touch something in /raid5 and delete one of partitions of sda5,sda6,sda7

This is how we can create a Raid device with level 1

***RAID 1***

#mdadm create /dev/md0 level=1 raid-devices=2 /dev/sda{5,6}

This is how we can create a Raid device with level 0

***RAID 0***

#mdadm create /dev/md0 level=0 raid-devices=2 /dev/sda{5,6}

Stopping mdadm

*Unmount the md0 before stopping mdadm

#mdadm stop /dev/md0

If you want to create additional devices[ie there exists a /dev/md0] you may need to add an "-a yes" option to the mdadm command

For example,

#mdadm create /dev/md1 -a yes level=0 raid-devices=2 /dev/sda{5,6}